The invention relates generally to x-ray tubes and, more particularly, to a method and apparatus of reducing high-voltage activity therein.
X-ray systems typically include an x-ray tube, a detector, and a rotatable assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object while the x-ray tube and detector are rotated about the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then transfers data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient positioned in a medical imaging scanner and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes typically include an anode having a high density track material, such as tungsten, that generates x-rays when high energy electrons impinge thereon. The anode structure typically includes a target cap and a heat storage unit, such as graphite, attached thereto. X-ray tubes also include a cathode that has a filament and a high voltage applied thereto to provide a focused electron beam. The focused electron beam comprises electrons that emit from the filament, typically tungsten, and are accelerated across an anode-to-cathode vacuum gap to produce x-rays upon impact with the track material. The anode and the cathode are typically positioned within a single volume that is maintained at a single vacuum level.
Because of the high temperatures generated when the electron beam strikes the track material, the anode assembly is typically rotated at high rotational speed. The anode typically includes a cylindrical rotor built into a cantilevered axle that supports the anode. An iron stator structure with copper windings surrounds the rotor and causes rotation of the anode via the rotor. The heat storage unit receives heat generated at the focal spot via conduction, and radiates the heat to the surrounding walls of the vacuum enclosure, where the heat is carried away by a coolant located outside the walls. The heat storage unit increases the heat capacity of the anode assembly, thus enabling longer and more frequent imaging sessions to be performed before the components of the x-ray tube overheat. The anode is typically mounted on a bearing assembly and rotated by an induction motor, and the bearing is typically placed within the vacuum region of the x-ray tube. The bearing assembly typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, therefore a solid lubricant such as silver is typically adhered to the balls to increase the life of the bearings.
Because of the high voltage requirements, the x-ray tube is susceptible to high voltage discharges, or “spits,” which interfere with operation of the x-ray system and lead to early life failure of the tube. Discharges occur as a result of high voltage operation in the presence of gases or particulate material within the x-ray tube (which raise its pressure), and the area surrounding the cathode is particularly susceptible to spit activity.
This phenomenon is exacerbated for a monopolar, or anode-grounded, tube design as compared to a bipolar design. When, for instance, a −140 kV voltage differential is maintained between the cathode and the anode and the tube is a bipolar design, the cathode may be maintained at, for instance, −70 kV, and the anode may be maintained at +70 kV. As such, the voltage differential between the cathode and the surrounding components at ground (and not the anode) is a net 70 kV. In contrast, for a monopolar design having likewise a −140 kV standoff between the cathode and the anode, the cathode accordingly is maintained at this higher potential of −140 kV while the anode is grounded and thus maintained at approximately 0 kV. Accordingly, the anode is operated having a net 140 kV difference with surrounding components within the tube. Thus, a monopolar tube design has increased voltage stand-off requirements for particularly the cathode, and therefore has increased sensitivity to gas and particulate in the area of the cathode. The high potential of the cathode in a monopolar design thus increases the propensity for high voltage activity in the cathode region as compared to a bipolar design. And such propensity is further exacerbated as gases and particulates collect within the vacuum region (thus raising its pressure) during the life of the tube.
Gases and particulates in an x-ray tube may emit from several sources. Such sources include, but are not limited to, the walls of the enclosure, the cathode components, and the anode components. For instance, the tungsten filament sublimates as a result of high temperature operation, thus causing tungsten particulate to emit into the vacuum region. Additionally, the walls of the enclosure, having a high surface area and typically an emissive coating thereon, emit gas into the vacuum region. The emission of gas and particulate matter is compounded as the operating temperature increases.
Furthermore, the anode itself typically has several sources from which gas and particulate matter may emit. Graphite in the anode, for instance, emits particulate and gas and is one of the worst offenders for causing high voltage activity. The bearing, likewise, emits particulate as a result of wear and is also a major source of particulate contaminants within an x-ray tube. Thus, by its operation, an x-ray tube typically includes a number of sources from which contaminant within the vacuum region may derive.
Commonly, the vacuum level in an x-ray tube is statically maintained and the vacuum region is evacuated at elevated temperature and sealed off. Gettering material is sometimes included in the vacuum vessel to aid in vacuum level retention. When the vacuum vessel is hermetically sealed via solid joints, the vacuum levels can be maintained so that the x-ray tube has a reasonably long operational life. However, if a constant gas source is included in the x-ray tube (e.g. a ferrofluidic rotating seal), additional vacuum pumping may be included to maintain the vacuum level during the tube life.
Typically, despite the various sources of contaminants, the vacuum level of the x-ray tube may be maintained by a single vacuum pump, such as an ion pump with a capacity of, for instance 8 l/s. However, such a pump is typically fairly bulky and is sized in order to properly pump the relatively large amounts of contaminants that emanate from primarily the anode and bearing in order to maintain the very high vacuum level around, for instance, the cathode.
The effect of gas and particulate emission from sources can be minimized to some extent by implementing design improvements or alternatives in an x-ray tube. For instance, an x-ray tube cathode is typically designed to have smoothed and rounded surfaces. And proper spacing between the anode, the cathode, and the surrounding components is typically maintained in the design to minimize the propensity for high voltage discharge. Such design activities represent practices that are developed with experience in the industry and may result in an increased tolerance of gas and particulate contamination within the vacuum.
As another example of gas and particulate emission reduction in x-ray tube design, the bearing may be placed outside the vacuum region by use of, for instance, a ferrofluid seal. Because the bearings may be positioned outside the vacuum region, they may be oil lubricated and may be designed to have greater load-bearing capacity than conventional x-ray tube bearings. A ferrofluid seal typically includes a series of annular regions between a rotating component and a non-rotating component. The annular regions are occupied by a ferrofluid that is typically a hydrocarbon-based or fluorocarbon-based oil with a suspension of magnetic particles therein. The particles are coated with a stabilizing agent, or surfactant, which prevents agglomeration of the particles in the presence of a magnetic field. When in the presence of a magnetic field, the ferrofluid is caused to form a seal between each of the annular regions. The seal on each annular region, or stage, can separately withstand pressure of typically 1-3 psi and, when each stage is placed in series, the overall assembly can withstand pressure varying from atmospheric pressure on one side to high vacuum on the other side.
The ferrofluid seal allows rotation of a shaft therein designed to deliver mechanical power from the rotor on one side of the seal to the anode on the other side. As such, the rotor may be placed outside the vacuum region and particulate generated due to bearing wear may be prevented from passing from the bearing to the vacuum region. However, while ferrofluid seals hermetically seal one side from the other, gas and water vapor may diffuse through the ferrofluid and into the high-vacuum region of the x-ray tube. In addition, the hydrocarbon-based or fluorocarbon-based oil used in the ferrofluid tends to evaporate or otherwise emit into the high-vacuum region of the x-ray tube as well. Accordingly, ionizable gases that transport through the seal or emit from the ferrofluid oil, when exposed to the high voltage environment of an x-ray tube, may lead to ionization failure of the x-ray tube, thus introducing a source of contaminant into the vacuum region.
Contaminants in an x-ray tube may also be minimized by use of proper cleaning and handling during the manufacturing process. However, despite even the efforts of special cleaning and processing of the components, gases and particulates may yet accumulate within the x-ray tube as a result of operation of the tube, thus increasing the tube pressure and causing increased high voltage activity that may lead to early life failure.
Therefore, it would be desirable to design an apparatus and method to minimize gas and particulate within an x-ray tube, thus improving the vacuum level surrounding the cathode of an x-ray tube and reducing high-voltage activity therein.